In LTE (Long Term Evolution) systems, a high throughput can be achieved through fast retransmission using an HARQ (Hybrid Automatic Repeat Request). The HARQ is performed in a MAC (Medium Access Control) layer, and in LTE standard, the number of HARQ processes managed at user equipment (UE) and a base station (evolved NodeB: eNB) is determined depending on cell duplex modes and so on. Also, if carrier aggregation is configured, an HARQ entity is configured for each cell or component carrier (CC) as illustrated in FIG. 1, and the respective HARQ entities maintain multiple HARQ processes.
In transmission and reception operations between user equipment and a base station, data is processed on a per HARQ process basis identified by an HARQ process number. Typically, asynchronous transmission is used in downlink communication, and the base station can transmit data from the HARQ processes at arbitrary timings as long as the timings are separated longer than a predetermined period (for example, 8 ms in FDD (Frequency Division Duplex)) from the previous transmission of the HARQ processes. Meanwhile, synchronous transmission is used in uplink communication, and the user equipment retransmits data from the HARQ processes at a predetermined cycle (8 ms) as illustrated in FIG. 2. Specifically, as illustrated, upon receiving an uplink grant from the base station, for example, the user equipment starts the HARQ process #0 to transmit uplink data from the respective HARQ processes #1 to #7. Typically, the HARQ process number is uniquely determined at the initial transmission timing of a PUSCH (Physical Uplink Shared Channel) and is not explicitly indicated to the base station.
According to the LTE standard, a transport block (TB) is decoded in accordance with a decoding procedure as illustrated in FIG. 3. First, upon receiving the transport block, at step S11, the MAC layer confirms the HARQ process number of the transport block and determines whether the transport block has been newly transmitted or retransmitted. If the transport block has been newly transmitted, the MAC layer decodes the transport block at step S12 and determines whether the decoding result is successful at step S13. If the decoding result is successful, the MAC layer forwards the decoding result to a disassembly and demultiplexing entity at step S14 and transmits an ACK at step S15. On the other hand, if the decoding result is not successful at step S13, the MAC layer stores data attempted for decoding in a soft buffer at step S16 and transmits a NACK at step S17.
On the other hand, if the transport block has been retransmitted at step S11, at step S18, the MAC layer determines whether the transport block has been successfully decoded before. If the transport block has been successfully decoded before, at step S19, the MAC layer transmits the ACK. On the other hand, if the transport block has not been successfully decoded before, at step S20, the MAC layer combines the transport block with data in the soft buffer. The MAC layer decodes the combined transport block at step S21 and determines whether the decoding result is successful at step S22. If the decoding result is successful, at step S19, the MAC layer transmits the ACK. On the other hand, if the decoding result is not successful, the MAC layer stores data attempted for decoding at step S23 and transmits the NACK at step S24.
In the fifth generation (5G) communication, three typical use cases as illustrated in FIG. 4 are assumed. Specifically, the three use cases are a use case where mobile broadband is further developed, a use case such as IoT (Internet of Things) where everything is connected to networks and a use case where highly reliable and ultra-low latency communication is achieved.